1. Field of the Invention
The present invention relates to a liquid crystal display device using plastic substrates and more particularly to a plastic substrate liquid crystal display device which is principally used as a display device for OA(Office Automation) appliances such as a note type personal computer, a word processor and a palm-top type personal computer.
2. Description of the Prior Art
A typical liquid crystal display device is comprised of a pair of plastic substrates, transparent electrodes, insulating films for protecting the electrodes (protective films), aligning or orientation films and a liquid crystal. A method of manufacturing this type of liquid crystal display device will be described with reference to FIG. 1. The manufacture method described herein is directed to a liquid crystal display device using a pair of plastic film substrates 3 and 3 as shown in FIG. 1
Firstly, plastic substrates 3 each have at both surfaces thereof hard coats (wear-proof coatings) 2 and 4 made of a resin of acryl system or a silicone system. The plastic substrate 3 is formed at one surface (provided with the hard coat 4) with an ITO electrode 5 made of indium oxide added with tin. Subsequently, an upper protective film 6 is coated on the surface of the ITO electrode 5 in a thickness of 500 to 1500 .ANG. by means of an offset printing machine which uses a commercially available material of silica coating ink (Ti-Si film manufactured by Tokyo Ohka).
Thereafter, the substrate 3 coated with the silica coating ink is pre-sintered at a temperature of 70.degree. C. for a few minutes. Through this, a solvent of high boiling point contained in the ink can be evaporated to some extent, with the result that the ink is increased in viscosity and prevented from flowing on the substrate 3.
In the liquid crystal display device using the plastic film substrates 3, thermal stability of the plastic film substrate 3 is insured at 200.degree. C. or less and correspondingly the sintering temperature of the silica coating ink must be limited to 200.degree. C. or less. However, by simply setting the sintering temperature to 200.degree. C. or less, the insulating capability and hardness of the protective film cannot be ensured.
To cope with this problem, the inventor of the present application has already proposed in Japanese Patent Application No. 3-71476 that in order to obtain low temperature sintering at about 200.degree. C., ultraviolet(UV) light is irradiated on the coating ink by means of a low pressure mercury vapor lamp to volatilize the solvent in the coating ink.
More specifically, ultraviolet light is irradiated on the pre-sintered silica coating ink for five minutes by using the low pressure mercury vapor lamp. The substrate 3 is then sintered at a sintering temperature of 170.degree. C. for one hour. Since in this phase the solvent of high boiling point has already been removed from the ink by the irradiation of ultraviolet light to make easy the association between molecules of the remaining solid substance, the dehydration condensation of the ink can be effected sufficiently even at the 170.degree. C. (less than 200.degree. C.) sintering temperature. In this manner, an upper protective film (top coat) 6 having sufficient insulation and hardness to protect the ITO electrode 5 can be formed without thermally changing the nature of the plastic film substrate 3. Thereafter, an aligning film 7 is formed on the surface of the upper protective film 6. A pair of thus prepared substrates 3 and 3 are arranged to oppose to each other, a liquid crystal 9 is filled while sealing the substrates by a seal agent 8 and polarizer plates 1 are attached, thus completing the manufacture procedure.
However, in conventional liquid crystal display devices including one manufactured by the above-described manufacture method, the aligning film is made of polyimide. The polyimide is created through condensation reaction of polyamic acid and a sintering temperature of 250.degree.to 350.degree. C. is needed for the condensation reaction. Therefore, even if the insulation and hardness of the protective film can be ensured at the heat-proof temperature of plastic being 200.degree. C. or less in the manner described above, the temperature is insufficient to provide the sintering temperature required for the conventional aligning film.
Accordingly, in the conventional liquid crystal display device, the shortage of this sintering temperature raised the most difficult problem in the procedure of making the plastic substrate liquid crystal display device, and this sintering temperature remarkably affected electrooptical characteristics of the device, i.e., the display quality and had a fatal influence upon the contrast, so that satisfactory optical characteristics could not sometimes be obtained.
Various techniques have hitherto been proposed to solve the above problems.
One of them is disclosed in Japanese Patent Application Laid-open No.56-52722. This literature describes a method of making a liquid crystal display panel wherein polymer or bismaleimide represented by the following repetitive unit (III) is reacted with benzil oxime to create polymer, the thus created polymer is dissolved in a solvent to provide a solution, the solution is coated on a plastic film panel substrate and dried and then sintered at a temperature of less than 150.degree. C. to form an aligning processing film, and then the film is subjected to a rubbing processing. By this making method, horizontal orientation performance which is physically and chemically stable at a temperature less than the heat-proof temperature of plastic film can be obtained. ##STR1##
Another conventional technique is disclosed in Japanese Patent Application Laid-open No.61-47931.
This literature describes that an aligning film mainly containing vinylidene chloride/vinyl chloride copolymer is formed in order to obtain an aligning film having excellent performance for uniformly orienting the liquid crystal. It also describes that the vinylidene chloride/vinyl chloride copolymer is dissolved in a cyclic ether type solvent such as dioxane or tetrahydrofuran to prepare a solution and the solution is coated, dried and subjected to heat treatment in accordance with a conventionally known method to form a film which in turn undergoes a rubbing processing. The heat treatment is effected at a temperature of about 60.degree.to 100.degree. C.
Still another conventional technique is disclosed in Japanese Patent Application Laid-open No. 60-66232. This literature describes that alkyd melamine polymer is used as an aligning film material which can exhibit excellent ability to orient the liquid crystal and which can be treated at so low a temperature that a plastic film substrate is allowed to be used. It also describes that especially, alkyd melamine polymer filled with silica is dried for about 30 minutes at a relatively low temperature, for example, 150.degree. C. to form an aligning film which has excellent ability to orient the liquid crystal and which is highly reliable from the standpoint of orientation capability and increase of current.